The genetic basis of complex, ecologically relevant traits that differ at the species level is not well understood in any organism. This proposal describes work that may contribute to our knowledge of both the basis of such traits and their evolutionary origin. Using two closely related insects, Heliothis subflexa and Heliothis virescens, this research examines the relationship between genetic divergence at the microevolutionary, intraspecifc level and the more sweeping genetic changes that accompany macroevolutionary phenomena such as speciation. Complex phenotypic differences between Heliothis subflexa and Heliothis virescens, a relatively recently diverged species pair, will be examined at the behavioral, physiological, and neurosensory level; the genetic basis of inter- and intraspecific variation will be explored using a QTL approach. The evolutionary trajectory of change in complex traits will also be examined: by conducting artificial selection on Heliothis virescens, a dietary generalist, to become phenotypically similar to Heliothis subflexa, a dietary specialist, this project will explore the potential for microevolutionary selection to contribute to species-level differentiation. By examining both the phenotypic changes that occur in response to selection and their genetic basis, this research will contribute to our knowledge of a poorly understood subject: the means by which intraspecific variation is transformed into fixed, complex differences between closely related species.